196 ON THE COLOURS OF THICK PLATES.
its image, so far as regards their forms and colours, they are not symmetrical so far as regards their intensities, but are decidedly more brilliant on the side of the image than on the side of the flame itself. That this is not due merely to the glare of the direct light, may be proved by holding a small object in front of the flame, so as to screen the eye from the direct light, when the rings, though better seen than before in the neighbourhood of the flame, are still much weaker than on the opposite side, if the distance of the flame from the axis is at all considerable. For the same reason, in the case of a plane mirror, when the luminous point is placed a good distance in front of the eye, so that the rings do not run out of the field of view, they cannot be traced throughout the whole extent if the angular distance between the luminous point and its image be too great, but only throughout a portion, more or less considerable, on the side of the image.
38. In the case of a concave mirror when the luminous point is not far from the centre of curvature, and the rings are viewed by an eye placed at no great distance off, the first factor in the expression for S2 (equation 38) is not large, and the angle of diffraction does not increase rapidly in passing away from the image. In the case of a plane mirror p = oo , and if we suppose c and h equal to what they were in the former case, or thereabouts, in order to make the two cases comparable in every respect except the curvature of the mirror, the factor in question, though larger than before, is still sufficiently small to prevent S from increasing very rapidly on receding from the image. Accordingly, in both these cases, the rings and bands are seen with brilliancy at a considerable angular distance from the image. But in the case of a convex mirror of considerable curvature p is negative, and not large, so that the factor in the expression for S2 becomes considerable, and accordingly the angle of diffraction increases rapidly on receding from the image. I found, in fact, that such a mirror was peculiarly ill suited for producing rings or bands, inasmuch as only a comparatively small portion of the system usually seen was visible, namely, the portion which lay in the immediate neighbourhood of the image.